Mobile holder for a wafer

ABSTRACT

The present invention provides a mobile holder for a wafer, which comprises a base element, a first fixing means and a second fixing means. The first fixing means is configured to allow a wafer to be fixed to the base element. The second fixing means is configured to fix the mobile holder to a support for said mobile holder.

FIELD OF THE INVENTION

[0001] The present invention relates to the field of holding devices forwafers and especially to the field of electrostatic holding devices forwafers.

BACKGROUND OF THE INVENTION AND PRIOR ART

[0002] In the production of many semiconductor components, the handlingof a wafer represents an important factor for a successful productionprocess. The production of chip cards and thin integrated circuits, forexample, necessitates that very thin wafers are handled, which have tobe transported and held without the risk of breaking.

[0003] Many semiconductor processes, such as the production of circuitelements for power electronics, additionally require back-surfaceprocesses, which comprise a deposition of solder or alloy layers on theback of the fully processed circuit wafer; in a subsequent alloying orsintering step of these layers, high temperatures occur, which mayexceed 400° C. The wafer holding device must then guarantee perfectfunctioning in the case of these ambient temperatures.

[0004] A known method of handling thin wafers is so conceived that thefront face of the wafer is provided with a protective foil, which istypically a polymer foil. The wafer is held by the carrier foil duringthe thinning sequence. The possibility of using this known methodcomprises wafers having a diameter of 6 inches and a thickness whichmust not exceed 100 μm. Handling and transport of wafers having athickness of less than 100 μm is prevented by the fact that the waferswill bend, whereby the risk of breaking increases significantly for thethin wafers. It follows that the method does not offer any possibilityof handling extremely thin wafers with comparatively large diameters.

[0005] Another method makes use of a carrier wafer to which the wafer tobe thinned is reversibly attached by means of an adhesive film which isadherent on both sides, e.g. a thermally strippable film. The thermallystrippable film can be stripped by subjecting it to a certaintemperature. The method is adapted to be used for wafers having athickness of not less than 20 μm, and also wafers having a largediameter can be held safely. The method is, however, disadvantageousinsofar as, due to the adhesive film used, which is typically a polymerfilm, the method is not suitable for back-surface processes involvingtemperatures above 130° C.

[0006] Furthermore, electrostatic holding devices, so-calledelectrostatic chucks, are known for taking up wafers by means ofelectrostatically generated holding forces and for holding these wafersduring a production process. The holding force can be generated by meansof a monopolar electrode or by means of bipolar electrodes. In the caseof a monopolar electrode, the wafer to be held serves as acounterelectrode and must therefore be connected to ground; during a dryetching process, for example, this connection to ground is obtained bythe conductive etch plasma.

[0007] The known electrostatic holding devices are, however,disadvantageous insofar as they have a solid structural design and are,typically, fixedly installed in a processing chamber. In addition, theknown electrostatic holding devices are dependent on an external powersupply. Hence, the known electrostatic holding devices can only be usedas holding devices for holding the wafer at one location, but theycannot be used for the purpose of transport, e.g. for taking up athinned wafer at the location where the thinning sequence has beenexecuted and transporting it to a second location where e.g. processingof the back surface will take place.

[0008] It follows that the handling of thin wafers, which necessitatehigh-temperature processing steps, such as sintering or alloying, isproblematic, since devices for handling such wafers are not available.

[0009] Furthermore, a support device is, at present, not available,which permits chips that have been subjected to a dicing process of thetype described e.g. in DE-19962763 A1 to be taken up and detachedselectively.

[0010] EP 0 552 877 A1 discloses an electrostatic chuck and a method ofexciting the same. The electrostatic chuck is part of a multi-chambersystem for processing integrated circuits. A wafer transport platesupports a wafer by means of an electrostatic holding device. The wafertransport plate itself is connected to a base of the multi-chambersystem by a four-bar connecting joint in such a way that the wafer canbe moved to and fro between various chambers by displacing and rotatingthe wafer support plate. The electrostatic holding device comprises adielectric base layer having arranged thereon electrode strips which areincorporated in a dielectric encapsulating layer. The dielectric baselayer is formed on the upper surface of the wafer support plate makinguse of conventional deposition, masking and etching steps.

[0011] EP 0 506 537 A1 discloses an electrostatic chuck comprising anintegral five-layer structure, which is suitable for holding andtransporting a semiconductor silicon wafer in a production process forelectronic components. A particularly characteristic feature of thiselectrostatic chuck is that the wafer held will be released immediatelywhen a voltage applied to electrodes of the chuck is switched off. It isthe object of the present invention to provide a concept for an improvedhandling of wafers.

SUMMARY OF THE INVENTION

[0012] In accordance with a first aspect of the invention, this objectis achieved by a mobile holder for a wafer, comprising: a base element;a first fixing means for electrostatically fixing the wafer to the baseelement, said first fixing means comprising a functional layer and anelectrode structure which is arranged between said functional layer andsaid base element, wherein the functional layer is formed such that thefirst fixing means is in an electrostatically active state without anysupply of voltage from outside;and a second fixing means for releasablyfixing the mobile holder to a wafer support.

[0013] In accordance with a second aspect of the invention, this objectis achieved by a method of handling a wafer comprising the followingsteps: providing a mobile holder for a wafer, said mobile holdercomprising a base element, a first fixing means used forelectrostatically fixing the wafer to the base element and including afunctional layer which is implemented such that said first fixing meansis in an electrostatically active state without any supply of voltagefrom outside, and a second fixing means for releasably attaching themobile holder to a wafer support; fixing the mobile holder to a wafersupport with the aid of the second fixing means; attaching a wafer tothe base element with the aid of the first fixing means; moving themobile holder from a first location to a second location; and detachingthe wafer from the mobile holder by actuating the first fixing means.

[0014] The present invention is based on the finding that a holder for awafer can be provided in that a first fixing means is used for fixing awafer to the holder, whereas a second fixing means is used forconnecting the holder to a support for the mobile holder.

[0015] One advantage of the present invention is that the holder can beattached to a movable support so that the holder is mobile.

[0016] Another advantage of the present invention is that various typesof supports can easily be used for the mobile holder.

[0017] A preferred embodiment of the present inventtion comprises amobile holder with a flat base element comprising a circular siliconwafer. On the front face of the base element an insulating layer isformed which has, in turn, formed thereon a first electrode and a secondelectrode in spaced relationship with one another. The insulating layerconsists of an electrically insulating material, such as silicon oxide.The electrodes are provided with respective connecting areas so thatthey can be connected to electric leads. The mobile holder isadditionally provided with a functional layer arranged over the fullarea of the first electrode and of the second electrode and consistingof a material which contains movable ions. In the preferred embodiment,the functional layer allows the mobile holder to be in an active statewithout being connected to an external voltage source. On top of theinsulating layer a cover consisting of an insulating material, such assilicon oxide, silicon nitride or silicon carbide, is formed as aprotective layer over the full area of the insulating layer.

[0018] In an alternative embodiment, the mobile holder comprises anactive layer consisting of a material having a high dielectric constant.

[0019] In a further embodiment, the mobile holder has a matrix electrodestructure in which individual electrodes can be controlled purposefullyand supplied with voltages so as to permit a pixelwise removal of chips.

[0020] Furthermore, another embodiment is provided with feed-throughleads for the first electrode and the second electrode, which extendthrough the base element so as to permit a connection of the electrodesfrom the back of the base element.

[0021] In one embodiment, the second fixing means constitutes the areaof the back of the mobile holder. In this embodiment, the support forthe mobile holder is a conventional vacuum wafer chuck, the mobileholder being attached to the vacuum wafer chuck by a vacuum appliedbetween the second fixing means and a fixing area of the wafer chuck.

[0022] In a further embodiment, the second fixing means is a margin ofthe mobile holder. In this embodiment, an attachment of the mobileholder to a known support is achieved in that the support mechanicallygrips the mobile holder on the margin thereof.

[0023] Furthermore, in the case of still another embodiment, the edge ofthe mobile holder constitutes the second fixing means, a known supportfixing the mobile holder by mechanically gripping the mobile holder atthe edge thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In the following, preferred embodiments of the present inventionwill be explained in detail making reference to the drawings enclosed,in which

[0025]FIG. 1 shows a cross-sectional view of a mobile holder accordingto one embodiment of the present invention; and

[0026]FIG. 2 shows a top view of a mobile holder according to oneembodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0027] According to FIG. 1, a mobile holder comprises a flat, circularbase element 1 having an insulating layer 2 formed on the front facethereof. The insulating layer 2 consists of an electrically insulatingmaterial such as silicon oxide or silicon nitride. A first electrode 3and a second electrode 4 are arranged on the insulating layer 2 inspaced relationship with one another. The first electrode 3 and thesecond electrode 4 preferably consist of a temperature-resistantelectric conductor, such as a metal having a high melting temperature,and are identical in area, as will be explained in the following withreference to FIG. 2.

[0028] The base element 1, which preferably consists of a semiconductormaterial, ceramics such as ALO₂, Kapton or SiC, has a diameter which isequal to the diameter of a wafer to be held or which is slightly largerthan the wafer to be held. In, a specially preferred embodiment, thebase element 1 is formed such that it has a diameter of approx. 150 mmand a thickness of approx. 680 μm; this corresponds to typicaldimensions of a standard wafer.

[0029] Due to the use of temperature-resistant materials for the baseelement 1, the mobile holder is suitable for high ambient temperaturesso that the mobile holder is suitable for holding process wafers duringtemperature-critical processes, such as an alloying or sintering ofmetal layers.

[0030] Silicon and other semiconductor materials show at hightemperatures an increase in their intrinsic conductivity, which causes adeterioration of the electrostatic holding force in the case of thevoltages applied to the electrodes, which are in the range of from 1,000V to 2,000 V. In processes involving high temperatures, e.g. in plasmaprocesses, ceramics and in particular SiC are preferred to semiconductormaterials as a material for the base element 1; due to the high thermalconductivity, SiC additionally guarantees that the wafer to be held iswell cooled in an advantageous manner. SiC has the further advantagethat the expansion coefficient does not essentially differ from that ofsilicon so that, especially in cases in which the wafer to be heldconsists of silicon, a use of SiC as a material for the base element 1will be advantageous.

[0031] The mobile holder additionally includes a functional layer 5which is arranged on top of the first electrode 3 and the secondelectrode 4 and in the spaces between these electrodes and which extendsover the whole surface of the wafer as a continuous layer. In thepreferred embodiment the functional layer 5 consists of a materialcontaining movable ions, such as borosilicate glass, or of a materialhaving a high dielectric constant, such as barium titanate, strontiumtitanate.

[0032] On top of the electrically insulating layer 5 a cover 6consisting of an insulating material, such as silicon oxide or siliconnitride, is formed over the full area of the insulating layer 5. Thecover serves to protect the functional layer 5 arranged therebelowagainst aggressive chemical substances.

[0033]FIG. 2 shows a top view of the mobile holder shown in FIG. 1, FIG.1 and FIG. 2 being not represented on the same scale. As can be seen inFIG. 2, the first electrode 3 has a first connecting area 7 and thesecond electrode 4 has a second connecting area 8, each of theseconnecting areas being arranged in the vicinity of the edge of themobile holder. The connecting areas 7 and 8 permit an electric voltageto be applied the respective associated electrodes. The arrangement ofthe connecting areas in the vicinity of the edge of the mobile holderallows an exposed mode of arrangement of these connecting areas when aprocess wafer is attached, in that said process wafer is attached to themobile holder in such a way that the flat portion of the wafer islocated in the region of the connecting areas. The term flat portiondescribes a portion in which the edge of the wafer, instead of followinga circular-arc shape, follows a straight line of a segment portion so asto indicate the direction of crystal growth. It follows that, in thisembodiment, electric leads for the connecting areas can be fixed to thefront face of the mobile holder.

[0034] In an alternative embodiment, feed-through leads for the firstelectrode 3 and the second electrode 4 extend through the base element 1so that, by means of connecting areas formed on the back of the baseelement 1, the electrodes can be electrically connected to leads. Leadsextending through the base element have the advantage that the processwafer can be positioning independently of the position of the flat.

[0035] In the embodiment shown in FIG. 2, the first electrode 3 and thesecond electrode 4 are, in addition, arranged in such a way that theelectrode 3 is essentially arranged in a first quarter circular segmentof the circle area of the base element 1 and in a second quartercircular segment located in diagonally opposed relationship with thefirst one, the electrode portions arranged in these quarter circularsegments being interconnected by a bridge which extends through thecentre of the circle area of the base element 1. Furthermore, theelectrode 4 according to FIG. 2 is essentially formed in the quartercircular segments in which the electrode 3 is not arranged. The twodiagonally opposed quarter circular segment portions of the electrode 4are interconnected via an elongate, curved connection area extendingalong the edge of the base element 1.

[0036] The first electrode 3 and the second electrode 4 are arranged insuch a way that they are spaced apart by an elongate interspace havingan essentially constant width. Furthermore, the electrodes 3 and 4 arespaced from the edges of the base element 1 in such a way that a marginis formed between the electrodes 3 and 4 and the edge of the baseelement 1, said margin being adapted to be used as a holding device forattaching the mobile holder to a support.

[0037] For attaching a wafer to be held, e.g. a very thin process wafer(20-100 μm), the front face of the mobile holder must be brought intocontact with or into close proximity to the side of the wafer to be heldat which the holder is to be attached.

[0038] In order to move the mobile holder to the take-up location, saidmobile holder is first fixed to a wafer support, e.g. a robot wafersupport. In a preferred embodiment, the margin formed between theelectrodes and the edge of the wafer serves as a fixing means for fixingthe wafer support; in this embodiment, the mobile holder has a slightlylarger diameter than the wafer to be supported so that the process waferto be taken up does not project into the marginal area of the mobileholder and can be fixed without any disturbance by an attachment meansfastened to the margin of the mobile holder. The mobile holder can befixed to an attachment means of the wafer support through pressurecontact or in some other suitable way, such as reversible bonding.

[0039] In a further embodiment, the edge of the base element 1represents the fixing means for attaching the mobile holder to a wafersupport. In this embodiment, the mobile holder is only held by pressurecontact between an attachment means of the wafer support and the edge ofthe mobile holder.

[0040] Furthermore, in another embodiment the back of the mobile holdermay have the function of a fixing means for attaching a wafer support.Alternatively, the mobile holder can also be transferred to a stationarychuck, such as an electrostatic chuck, said chuck being secured to theback of the mobile holder.

[0041] For facilitating a transfer of the mobile holder, said mobileholder may additionally be provided with suitable adjustment and sensorelements.

[0042] As has already been mentioned, the shape and the size of themobile holder are preferably of such a nature that said mobile holderdiffers only insignificantly from a standardized shape and size forprocess wafers; even wafers having a large diameter and a smallthickness can be held easily by the mobile holder. The standardizedshape of the mobile holder makes it possible that the mobile holder canbe stored in standardized storage devices for wafers, e.g. in racks orhurdles, and that it can be taken up from said racks by standard wafersupports, such as robot handling devices, and transported to desiredlocations. In view of the fact that it is possible to use the holdingand gripping devices provided as part of the standard equipment in thefield of semiconductor production, the mobile holder according to thepresent invention represents a reasonably-priced system, which, inaddition, can easily be attached to an arbitrary type of wafer support,a circumstance which essentially facilitates integration in asemiconductor production process.

[0043] When the mobile holder has been fixed to the wafer support, themobile holder can be moved by means of said wafer support to the processwafer to be held.

[0044] In order to make it possible that the wafer to be held is held onthe mobile holder, the mobile holder must be in an activated state inwhich the electrodes 3 and 4 have applied thereto opposite voltageswhich are sufficiently high for fixing a wafer. Due to the provision ofthe functional layer 5, the mobile holder is implemented such that itwill maintain an activated state also without any external voltagesource. This is achieved by the use of suitable materials for thefunctional layer 5. The material employed so as to be able to use thefunctional layer 5 as a storage layer for storing electrostatic chargesin an activated state is preferably a dielectric material with movableions, such as borosilicate glass, which contains movable sodium andpotassium ions. Alternatively, a material having a high relativedielectric constant can be used.

[0045] The use of the above-mentioned materials allows a quasi-permanentcharge shifting according to the known capacitor effect. The mobileholder is only connected to an external voltage source for the purposeof charging and is in an activated state also after having beenseparated from the external voltage source; in this activated state, avoltage which is sufficiently high for holding the process wafer isapplied between the first electrode 3 and the second electrode 4. In thecase of prolonged holding times, the mobile holder can be connected viathe connecting areas 7, 8 to the external voltage source for the purposeof “refreshing” the charges.

[0046] In the activated state, an electric field is produced by theoppositely charged electrodes 3 and 4, said electric field causing in awafer to be supported, which is in contact with or in close proximity tothe front face of the mobile holder, a charge transfer, whereby a forceis produced which holds the wafer to be held on the front face of themobile holder. Depending on the respective field configuration, positiveor negative charges are generated on the surface of the wafer to besupported. It follows that a wafer held on the mobile holder has asurface charge distribution comprising essentially areas with positiveand negative charges which correspond to the arrangement of theelectrodes 3 and 4. In other words, an electrode 3 produces e.g. apositive charge on the surface of a wafer to be held in the area locatedopposite to said electrode 3, whereas the electrode 4 produces anegative charge on the surface of a wafer to be held in the area locatedopposite to said electrode 4. It follows that the arrangement of theelectrodes 3 and 4 in segments of a circle area has the advantage that,by connecting an external voltage source, it is possible to reverse thepolarity of or to disable one of the two electrodes so that a holdingforce can be reduced and switched off.

[0047] The wafer holder of above-described embodiment can be used in aparticularly advantageous manner for dicing chips in connection with dryetching. This kind of use will be explained in the following.

[0048] The process wafer is here first prestructured with trenches fordicing the chips on the front face of the process wafer. In order topermit a removal of diced chips, the trench structure must correspond tothe arrangement of the electrodes. In other words, the above-describedembodiment of a mobile holder with an electrode arrangement in quartercircular segments can be used for dicing a wafer intoquarter-circular-segment chips, the structured trenches subdividing thewafer into quarter circular segments.

[0049] The depth of the structured trenches is chosen in accordance withthe desired residual thickness of the wafer, which the wafer is intendedto have after a thinning and dicing process; in the case of e.g. thinfilm chips, this residual thickness is in the range of from 40 to 80 μm.

[0050] For producing thin chips, the thinning process is first carriedout in the usual way, e.g. by grinding and stress-relief etching, untila thickness of approx. 100 μm has been reached. The prestructuredtrenches are still closed at that time. Subsequently, the wafer issubjected to a thinning process executed e.g. in a vacuum plasmachamber. The wafer can be transported into the plasma chamber eitherwith the mobile holder or with a known holding device and in said plasmachamber it can be fixed to a stationary holding device, such as anelectrostatic chuck.

[0051] Alternatively, the wafer may also remain on the mobile holder forcarrying out the thinning in the vacuum plasma chamber.

[0052] In the plasma chamber, the wafer is then thinned by dry etchingthe back until the desired thickness of approx. 40 to 80 μm has beereached, whereby the trenches are opened from the back. By opening thetrenches, the wafer will automatically be diced into individual chips.Following this, the mobile holder, which can be kept e.g. in a rack inthe vacuum plasma chamber, is fixed to a movable support, e.g. a robotwafer support. The fixing to a suitable support can be carried outeasily, since the mobile holder has essentially the shape and the sizeof a wafer, the support seizing the holder e.g. at the edge or themargin of the holder.

[0053] Subsequently, the front face of the mobile holder is moved intoclose proximity to the diced chips so that the segment portions of thefirst electrode 3 and of the second electrode 4 are located in opposedrelationship with the respective chips. For effecting a precisealignment a suitable adjusting device can be used. Furthermore, sensorscan be provided on the mobile support so as to allow exact alignmentwith respect to the chips.

[0054] Following this, the stationary electrostatic chuck releases thediced chips, which are then taken up by the mobile holder. The mobileholder is then transported to a discharge location by the movablesupport. At the discharge location, the diced chips attached to themobile holder are selectively removed from said mobile holder byreversing the polarity of electrode 3 or electrode 4.

[0055] The polarity of the electrodes is reversed by applying apolarity-reversal voltage with opposite polarity to the connecting areaof the respective electrode. If, for example, the polarity of electrode3 is reversed, the surface charge in chips located opposite to saidelectrode 3 will be diminished, whereby the holding force will bereduced and the respective chips will be detached from the mobile holderwhen a lower limit of the holding force has been reached.

[0056] The wafer having a thickness of approx. 100 μm is then fixed tothe mobile holder in such a way that the quarter circular segments ofthe wafer, which correspond to the chips to be diced, are located inopposed relationship with the quarter circular segments of theelectrodes 3 and 4 of the mobile holder.

[0057] The described embodiment of the present invention provides thepossibility of selectively detaching two of the fourquarter-circular-segment chips by reversing the polarity of one of thetwo electrodes. In order to permit a selective detachment of each of thefour quarter-circular-segment chips, an electrode structure of fourseparate quarter-circular-segment electrodes is provided in the case ofa further embodiment, each of these quarter-circular-segment electrodesbeing provided with a connecting area for applying a voltage. First, theelectrodes are activated by switching the leads in a suitable mannerduring the activation according to the embodiment of FIG. 2, so that tworespective diagonally opposed electrodes will have the same polarity.When the chips have been taken up, one or a plurality of thequarter-circular-segment chips can be detached selectively bypurposefully reversing the polarity of the four quarter-circular-segmentelectrodes, whereas the rest of the quarter-circular-segment chips isstill held by the mobile holder. The concept of independent electrodes,which define in the above-described embodiment an electrode structure inthe form of a 2×2 matrix, can, in a further embodiment, be extended to alarger number of electrodes which are arranged in a matrix having mcolumns and n lines.

[0058] By providing the electrode structure in the form of a matrix,individual chips can be removed “pixelwise” by reversing the polarity ofrespective electrodes of the matrix. In this embodiment, the mobileholder can be used for dicing a plurality of chips from a wafer.

[0059] Furthermore, the mobile holder according to the present inventioncan be used for selectively taking up diced chips from a plurality ofchips by controlling the connection voltages in a suitable manner.

1. A mobile holder for a wafer, comprising: a base element; a firstfixing means for electrostatically fixing the wafer to the base element,said first fixing means comprising a functional layer and an electrodestructure which is arranged between said functional layer and said baseelement, wherein the functional layer is formed such that the firstfixing means is in an electrostatically active state without any supplyof voltage from outside; and a second fixing means for releasably fixingthe mobile holder to a wafer support.
 2. A mobile holder according toclaim 1, wherein the electrode structure comprises a first electrode anda second electrode.
 3. A mobile holder according to claim 1, wherein theelectrode structure comprises more than two electrodes.
 4. A mobileholder according to claim 3, wherein the more than two electrodes arearranged in a matrix.
 5. A mobile holder according to claim 1, whereinthe first fixing means is provided with a connecting area for connectingthe electrode structure to electric leads.
 6. A mobile holder accordingto claim 4, wherein the connecting area is arranged on a front face ofthe mobile holder.
 7. A mobile holder according to claim 4, wherein aconnecting area is arranged on the back of the base element, said mobileholder being provided with a feed-through lead for connecting theconnecting area to the electrode structure.
 8. A mobile holder accordingto claim 1, wherein the functional layer consists of borosilicate glass,barium titanate or strontium titanate.
 9. A mobile holder according toclaim 1, wherein the base element consists of a semi-conductor material,Kapton or ceramics.
 10. A mobile holder according to claim 1, whereinthe base element has a front face and a back, the front face havingarranged thereon the first fixing means and the back of the base elementbeing implemented as an area and representing the second fixing means sothat the mobile holder is adapted to be releasably connected to a vacuumchuck as a wafer support.
 11. A mobile holder according to claim 1,which comprises as a second fixing means a marginal area into which awafer held by the first fixing means does not project, said marginalarea being implemented for fixing to the wafer support.
 12. A mobileholder according to claim 1, wherein the base element comprises as asecond fixing means an edge which is implemented such that the mobileholder is adapted to be releasably fixed to the wafer support simply bymeans of pressure contact between the wafer support and said edge.
 13. Amobile holder according to claim 1, which comprises a front face and aback, the front face being adapted to have the wafer attached theretoand the back being implemented as a second fixing means for fixing to anelectrostatic chuck as a wafer support.
 14. A mobile holder according toclaim 1, which has a shape of such a nature that it is adapted to bestored in wafer storage devices.
 15. A method of handling a wafercomprising the following steps: providing a mobile holder for a wafer,said mobile holder comprising a base element, a first fixing means usedfor electrostatically fixing the wafer to the base element and includinga functional layer which is implemented such that said first fixingmeans is in an electrostatically active state without any supply ofvoltage from outside, and a second fixing means for releasably attachingthe mobile holder to a wafer support; fixing the mobile holder to awafer support with the aid of the second fixing means; attaching a waferto the base element with the aid of the first fixing means; moving themobile holder from a first location to a second location; and detachingthe wafer from the mobile holder by actuating the first fixing means.